The present invention relates to a method of preparing a hybrid of polyvinylimidazole and silica, particularly a method of preparing a hybrid of polyvinylimidazole and silica by a sol-gel process.
Organic-inorganic hybrid materials have wide applications due to their excellent transparency, wear resistance, and heat resistance, etc. The applications include coating materials, optical materials, electronic materials and biomedical materials. Such materials, due to their organic/inorganic characteristics, show excellent features when being applied in products containing both the organic and inorganic components therein.
Researches have been carried out prevalently on nitrogen-containing polymers as electron donors. A major reason for such intensive attention lies on their wide applications which include: destabilizing anionic gel in discharges and water purification [S. Neyret, L. Ouali, F. Candau and E. Pefferkorn, J. Colloid Interface Sci., 176, 86, (1995)], electrophoretic deposition [B. Cabot and A. Foissy, J. Mater. Sci., 33, 3945, (1998)], curing agent [J. M. Barton, I. Hamerton, B. J. Howlin, J. R. Jones and S. Liu, Polymer, 39, 1929, (1998)], activation of electrodes [K. Kham, A. Deratani and B. Sebille, New J. Chem., 16, 505, (1992)], recovery of minute metal ions [C. Wu, W. Chen, J. Lee, J. Colloid Interface Sci., 183, 236, (1996); L. Martinot, D. Leroy, C. Jerome and O. Leruth, J. Radioanal. Nucl. Chem., 224, 71, (1997)] and anti-corrosion coating [J. Jang, I. Jang and H. Kim, J. Adhesion Sci. Technol., 12, 323, (1998); J. Jang and H. Ishida, Corros. Sci., 33, 1053, (1992) B. Mouanda, Polymer, 38, 5301, (1997)], etc. Since the synthesis of polyvinlyimidazole (PVI) is rather simple [B. Popping, A. Deratani, B. Sebille, N. Desbois, J. M. Lamarche and A. Foissy, Colliods Surf., A 64, 125, (1992)] and PVI can be formed with a permanent positive charge density by controlling the ionization thereof [R. Bohmer, W. H. A. Heesterbeek, A. Deratani and E. Renard, Colliods Surf. A99, 53, (1995)], applications of polyvinlyimidazole have attracted many researchers.
Silica, having a very low water absorbancy (xcx9c0% by weight) and thermal expansion coefficient (xcx9c0.5 ppm/K), is an ideal inorganic material for providing polymers with such properties.
According to the literature reports, there are two methods for synthesizing a hybrid of tetravalent PVI and silica: (1) coating a tetravalent PVI compound with silica [N. Thuaud, B. Sebille, A. Deratani, B. Popping and C. Pellet, Chromatographia, 36, 373, (1993)]; and (2) carrying out a cross-linking reaction of PVI adsorbed on the surface of silica [N. Thuaud, G. Lelievre, A. Deratani and B. Sebille, Eur. Polym, J., 33, 1015, (1997); N. A. Taleb, M. C. Millot and B. Sebille, J. Chromatogr. A, 776, 45, (1997)]. However, these hybrids do not form any chemical bonding between the polymer and silica, not to mention forming a stable covalent bonding. Therefore, these hybrids have defects such as a poor thermal stability, or even a problem of phase separation, thereby further affecting the mechanical properties of the hybrids produced.
An objective of the present invention is to provide a method of preparing a hybrid of PVI and silica having covalent bonding therebetween. The invented method can prepare a PVI-silica hybrid with an excellent thermal stability.
The present invention discloses a method of preparing a hybrid of polyvinylimidazole and silica (PVI-silica hybrid), which comprises the following steps:
a) copolymerizing vinylimidazole and a vinyl-containing alkoxysilane in an organic solvent to form a modified polyvinylimidazole having the following formula I; 
wherein m is an integer of 10-100, n is an integer of 5-30, R1 is C1-C6 alkylene, R2 is C1-C4 alkoxy, R3 and R4 independently are hydrogen, C1-C4 alkoxy or C1-C4 alkyl, and R5 is hydrogen or C1-C4 alkyl; and
b) adding water and an organic solution of tri- or tetra-(C1-C4 alkoxy)silane into the resulting copolymerization mixture from step a) to perform hydrolysis and condensation reactions of the modified polyvinylimidazole (I) and the tri- or tetra-alkoxy silane to form a hybrid of polyvinylimidazole and silica comprising the following network structure 
wherein R6 is hydrogen or C1-C4 alkyl.
Preferably, said vinyl-containing alkoxysilane used in step a) is [3-(methacryloxy)propyl]trimethoxysilane.
Preferably, said tri- or tetra-(C1-C4 alkoxy)silane used in step b) is tetra-(C1-C4 alkoxy)silane. More preferably, said tetra-(C1-C4 alkoxy)silane is tetramethoxysilane.
In step a) of the method disclosed in the present invention, preferably the vinylimidazole and [3-(methacryloxy)propyl]trimethoxysilane are used in a molar ratio from 99:1 to 80:20; and a free radical initiator, such as azoisobutyronitrile (AIBN), is used to initiate the copolymerization reaction.
In step b) of the present invention, preferably the tetramethoxysilane and said modified PVI (I) are used in a weight ratio from 35:65 to 10:90; the water to the tetramethoxysilane are used in a mole ratio from 3:1 to 10:1; and an acid, e.g. HCl, is added into water as a catalyst, where the amount of the acid used is 1-5% by weight, based on the total weight of the water and the acid.
Preferably, the organic solvent used in step a) and an organic solvent suitable for preparing the organic solution used in step b) in the present invention are selected from a group consisting of tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), N,N-dimethylacetamide (DMAc), dimethylformamide (DMF), dimethylsulfoxide (DMSO), m-cresol, pyridine (Py), methyl chloride, ethyl chloride and a mixture thereof. More preferably, the organic solvent used in step a) and the organic solvent suitable for preparing the organic solution used in step b) in the present invention is tetrahydrofuran (THF).